1. Field of the Invention
The present invention relates in general to a method for the degradation of phosphate esters, particularly neurotoxins and pesticides, by metallocene derivatives, more particularly molybdocene and tungstocene derivatives. The degradation of organophosphorus compounds such as phosphate esters are necessary to dispose of the unwanted phosphate esters, for purposes including but not limited to destroying existing supplies of phosphate esters and remediating contaminated soil and water.
2. Problems in the Art
Phosphate esters include numerous chemical warfare agents such as VX, pesticides and insecticides, further including paraoxon and parathion. Each of these compounds includes a phosphate ester bond, and irreversibly blocks a serine hydroxyl group within the enzyme acetylcholinesterase by phosphorylation, resulting in a disruption of a cell's neurologic function. VX (O-ethyl-S-[2-(diisopropylamino)ethyl]methylphosphonothioate) is a lethal phosphonothioate neurotoxin (LC50=0.135 mg/kg) with the formula

Symptoms of exposure to VX include coughing, difficulty breathing, sweating, vomiting, urination/defecation, headache, tremors, unsteadiness and confusion, ultimately progressing to death. The United States has a stockpile of thousands of tons of VX that must be destroyed to comply with the Chemical Weapons Treaty of 1997. In addition, Russia is also known to possess quantities of VX.
Numerous phosphate esters are also used as pesticides which are toxic to unintended targets such as mammals. Examples include Paraoxon (O,O-diethyl-O-p-nitrophenylphosphoric acid) and Parathion (O,O-diethyl O-4-nitrophenyl phosphorothioate) with the respective formulas

Phosphate esters used as pesticides pollute soil and water with which they come in contact. The creation of phosphate esters for chemical warfare and use as pesticides results in the need for a safe and effective process of degradation in order to completely eliminate the compounds without persistent toxic environmental and medical effects. Much of the groundwork on degrading phosphate esters as nerve agents has been done on the pesticides paraoxon and parathion.
VX is degraded on a large scale by hydrolysis with concentrated aqueous sodium hydroxide, such as in 0.1 M NaOH, resulting in competing cleavage of the P—S and P—O esters, with approximately 87% P—S bond cleavage and 13% P—O bond cleavage (FIG. 1). This is problematic because the byproduct of the P—O bond cleavage, S-[2-(diisopropylamino)ethyl]methylphosphonic acid, has a toxicity comparable to VX and requires additional steps such as oxidative pretreatment for destruction. Caustic neutralization at 90° C. (16.6 wt. % VX, 8.8 wt. % NaOH, 74.6 wt. % H2O) produces a similar ratio of bond cleavage, but allows S-[2-(diisopropylamino)ethyl]methylphosphonic acid to be broken down concurrently producing methyl phosphonic acid and thiolamine. However, this process requires specific control over both the pH and temperature of the reaction to ensure no byproducts are produced.
There is a need for a method to selectively cleave the P—S bond of the phosphate ester VX to eliminate the toxic byproducts of its degradation, so as not to require further degradation. Various additional aqueous compounds have been used in the degradation of VX, but are either unsuccessful at selectively cleaving the P—S bond or present commercial difficulties in their ability to be used in mass quantities. For example, aqueous potassium peroxymonosulfate selectively cleaves to the P—S bond in VX. However, the solubility of potassium peroxymonosulfate is limited at low pH and the oxidant decomposes at any pH above 5. Alternatively, the use of potassium peroxymonosulfate in polar organic solvents generates a toxic diphosphonate as a major byproduct.
Magnesium oxide and alumina are reported to degrade VX, but these methods are limited to surface chemistry. Other degradation methods for phosphonothioates include incineration and oxidation with peroxides. Incineration is a politically unpopular degradation method. Alternatively, hydrolytic degradation of phosphonothioates lacks selectivity and results in both P—O and P—S degradation pathways, resulting in toxic byproducts.
The known methods to degrade phosphate pesticides include hydrolysis by microorganisms, degradation or hydrolysis by Cu (II), Hg(II) and clays, surface catalyzed hydrolysis by Al2O3, TiO2 and FeOOH (goethite), and hydrolysis by Rh (III) and Ir (III) coordination complexes that are overly expensive and the hydrolysis by the molydocene derivative, bis(η5-cyclopentadienyl)molybdenum (IV) dichloride (Cp2MoCl2). Cp2MoCl2 similarly hydrolyzes dimethyl phosphate, a phosphate ester that mimics the diester functionality of DNA.
The art is the field of the invention clearly illustrates an immediate need for a more selective degradation method for VX and related neurotoxins, as the safety and environmental impact of their degradation are of great concern to the public. There further exists a need to provide a degradation method for VX and related neurotoxins that does not produce toxic products. There is also a need for a degradation method for VX and related neurotoxins that operate at room temperature. There exists further a need for a degradation method for phosphate esters under conditions neither very acidic nor very basic, such as a pH between 4 and 10. There exists further a need to provide a faster method to degrade phosphate esters including neurotoxins and pesticides. More generally, there is a need for a fast and selective method to degrade phosphate esters. The present invention builds on the past work with metallocene catalysts to create a viable method of degrading phosphate esters.